Micromouse Meeting #4Lecture #3

Sensors

Signals

We deal with voltage signalsSensors convert environment data to electrical signals

Output: Voltage Input: Time/Distance/Whatever

Move Receiver around

0 2 4 6 8 10 12 14 1601234567

Reading

Distance (cm)

Vol

tage

(V

)

Use sensors for:Wall Detection Accurate

TurningCell Counting

*Covered in Meeting #3 Powerpoint

Rotary Encoder

IR LED Sensors

Gyro

High Level Diagram

Used to measure distance traveled

Two major flavorsLED emitter/receiver pair with optically marked diskHall effect sensor with magnetically marked disk

Rotary Encoder*Covered in Meeting #3 Powerpoint

One emitter/receiver pair can be used to detect walls in one direction

Use infrared light to avoid visible ambient light interference

Infrared LED Sensors

Need to know if there are

walls around mouse

Emitter emits light

Light reflects off wall, if

there is a wall

Receiver measures light

intensity

Determine presence of

wall and distance to it

LEDs emit light with luminance dependent on voltage/current.They work the other way too

Sharp sensorsModel GP2Y0A21YK

Pre-made and assembledVery easy to use, but they are bulkyProvides single analog output to useSlow response time

Receiver TypesCustom sensorsEmitter-Receiver pair requiredCan be specially chosen for your

needsRequires an IR LED Driver circuit:

Darlington Driver ICNarrow emission angle is necessary

in LEDsPotentially more accurate, but can be

harder to calibrate

Choosing LED Emitters/Receivers

Two Important Characteristics:

Emission Angle Power Density

How sharp of an angle the LED emits light at

How brightly the LED emits

Look for narrow emission angle Maximizes power efficiency Maximizes signal amplitude received by the

receiverAlso indicated by “viewing angle”

Choosing a LED Emitter

Check datasheet for directivity graphs, which show intensity vs. angle

This angle should be small

Power Density Measures light intensity/solid angle In data sheets, often measure in mW/srHigher means more light emitter/better

Match emitter wavelength with the receiver’s most sensitive wavelength

Receivers also have directivity – minimize this Reduces interference from other IR light

sources Most common wavelengths are 850 nm

and 950 nm Check datasheets for this information

Choosing a LED Receiver

This emitter emits most at 940 nm

This receiver is most sensitive to 950 nm light

IR LED emitters and receivers are often sold together and are wavelength-matched already

Receivers also have directivity – Look for narrow ellipses

Need at least 3 pairs to detect walls in front and sides

4 or more is recommended for calibration2 to detect side walls2 pointing front to detect front walls

and front wall alignmentUsed to straighten the mouse

LED Emitter/Receiver

Get distance to wall by reading the voltage output of the receiver

Read the voltage output with MCU

Relate Voltage output with distanceLEDs are nonlinearFind the relation

experimentally

IR LED Data

1 2 3 4 5 6 7 8 9 10 11 12 13 14 150

1

2

3

4

5

6

7

Wall Sensor Reading

Distance (cm)

Vol

tage

(V

)

Optional, but highly recommendedEncoders can be used to measure angular velocity insteadBut they are less accurate and susceptible to wheels slipping

Used to measure rotation of the mouseNeeds stable power source

Otherwise, lots of noise generated

GyroscopeGyros output angular velocity about a

axis

Mouse needs to turn a certain amount and

begins turningGyro reports angular

velocity to MCUIntegrate to get current

turn angleMouse continues turning until desired turn angle

is achieved

SMD style SMD on breakout board style

Only need to measure one axis

Choosing a Gyro

Analog or digital output: MCU can handle either

Most important characteristic: RangeTypically measured in degrees/secondWhat range you need depends on how

fast your mouse spins+- 1000 degrees/second is plenty

Analog output:When not turning, voltage is half of

maximumTurning clockwise/counterclockwise

will change the output positively/negatively, depending on specific gyro

Digital output:Uses a serial scheme such as I2C or

SPISame output as analog output, but

numbers are encoded digitally (bits)

Gyro Data

For this analog gyro:

Turning clockwise increases voltage

Turning counterclockwise decreases voltage

Gyros measure angular velocityIntegrate angular velocity to get angular position (which is

more useful to know)Gyro output is recorded as discrete samples, so the integration

is a summation

Gyro Data Processing

Relate voltage output summation with angleCan be done experimentally

Gyro Data Processing

Non-ideality: Gyro driftGyros do not measure angular velocity perfectlyIntegration of the velocity result in an error that

increases linearly over timeMeasure the error and subtract it out

We’ll cover how to do this next time

Algorithms!EAGLE tutorial next week

Learn how to design printed circuit boards in EAGLEHosted by our Projects Manager Julian BrownNov. 14, 6 PM, location TBD

Future Things